Energy

Energy – the stored ability to do work.

There are many forms of energy and each form be transferred from one type to another.

Types of Energy

Below is a list of different forms of energy:

• Chemical
• Elastic Potential
• Electrical Potential
• Electromagnetic
• Gravitational Potential
• Heat
• Internal
• Kinetic
• Light
• Nuclear
• Sound

In general energy is often either kinetic energy or potential energy.

Kinetic energy is the energy an object has due to its motion and potential energy is the energy an object stores that has the potential to do work.

The Conservation of Energy

The Conservation of Energy – in a closed system energy cannot be created or destroyed; it can only be transformed from one type to another.

This means that energy is never lost, the total energy of an isolated system will remain constant. 

For example, if 60J of electrical energy is supplied to a bulb and only 15J of that energy is transferred into light energy we know, from the conservation of energy, that the remaining 45J of energy must have been transferred to another type of energy.

In the case of a bulb this energy is wasted as heat energy to the surroundings. Although this isn’t useful energy, no energy has been lost.

The energy transfers of a process like this can be represented using a Sankey diagram.

Sankey Diagrams

Sankey diagrams are a type of flow diagram used to show energy transfers, in which the size of an arrow represents the amount of energy.

Drawing a Sankey Diagram

Sankey diagrams should be drawn to scale, so it is often easiest to draw them on square paper.

This allows you to easily come up with a scale, for example 1 square could equal 1J. Alternatively, if you don’t have square paper, you could use lengths, such as 1cm is equal to 1J.

Once you have decided on your scale follow the below steps to draw your Sankey diagram.

• Step 1 – draw a line to represent your input energy, this is the start of the diagram which can now be split up into arrows, each representing the different forms of output energy
• Step 2 – plot the useful energy outputs as straight arrows, remembering to stick to your scale
• Step 3 – plot the wasted energy outputs as bent arrows pointing downwards
• Step 4 – label the input energy and all of the output arrows

Each energy output should have a different arrow. So if you had two types of useful energy output and 3 types of wasted energy output, you should have 5 arrows in total; two straight output arrows and 3 bent output arrows.

Due to the conservation of energy the total input energy should equal the sum of all the output arrows. If you have some energy missing, there must be another form of wasted/useful energy.

Example

Let’s take a look at the Sankey diagram for the bulb mentioned above:

From the scale we can see that each square represents 15J of energy. As the electrical (input) energy is 60J this is represented by 4 squares.

The 15J of useful light energy is shown by the straight arrow (with a width of 1 square) and the 45J of wasted heat energy is shown by the bent arrow pointing downwards (with a width of 3 squares).

As the diagram has been drawn to scale we can see that the size of the arrows is proportional to the amount of energy, with the light energy having the smaller arrow.

From the diagram we can also see the conservation of energy; with 60J of input energy and a total sum of 60J of output energy.

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